robotbuilder: remove Xt and sXp types from model

since they are not used anymore
robotbuilder: remove mass from the model

because it is not used and is available in the Spatial::Inertia
anyway.
robot builder: more input validation
robotbuilder: remove replacements_ member
robotbuilder: remove unnecessary is_initialized checks
robotbuilder: check name & libname
robotbuilder: remove unused member variable